An acoustic characteristic common to virtually all automotive sound systems is a rapid rate-of-change of interaural phase difference (IPD) typically equal to at least 90 degrees per octave to a maximum of 180 degrees at approximately 300 Hz as measured at the left and right side seat positions. The above described 180 degree maximum phase shift occurs over a "narrow-band region" between 150 Hz and 300 Hz therefore having a bandwidth on the order of 150 Hz. The above IPD anomaly constitutes a non-linear phase shift, as opposed to a linear phase shift or constant time delay that would be expected due to the difference in distances between each stereo speaker and a listener located at either the left or right side seat position.
The IPD occurring over such narrow-band region undergoes increasing phase lag as function of frequency at one side of a vehicle and undergoes increasing phase lead as function of frequency at the opposite side of the same vehicle. It should be noted, however, that IPD, in terms of phase correlation, remains nominally one at frequencies below such narrow-band region at both sides of such vehicle, and remains nominally minus one for frequencies above such narrow-band region up to approximately 1,000 Hz at both sides of the vehicle. The phase correlation is not predictable and therefore essentially zero at both sides of the vehicle above approximately 1,000 Hz due to acoustic reflections and standing waves; however, human hearing is not sensitive to IPD or phase correlation at and above 1,000 Hz. It follows that a signal process that provides a rapid rate-of-change of phase to a maximum of approximately 180 degrees between the stereo channels in the above described narrow band region introduces a correlation correction coefficient of plus one at frequencies below 150 Hz and minus one at frequencies above 300 Hz. When such correction coefficient is multiplied by the uncompensated phase correlations occurring at each side of the vehicle, the resulting corrected phase correlation at one side of the vehicle equals approximately one for all frequencies below 1,000 Hz, and the resulting corrected phase correlation at the opposite side of the vehicle equals approximately one for all frequencies below 1,000 Hz exclusive of the narrow-band region. Since the narrow-band region occupies a bandwidth of approximately 150 Hz, the percentage of non-corrected bandwidth relative to the corrected bandwidth in the phase sensitive region between 100 Hz and 1,000 Hz at such opposite side of the vehicle equals less than 17 percent, which is generally inaudible in terms of effect upon stereophonic imaging. It follows that the above signal process of the present invention provides substantially improved imaging at both sides of a vehicle simultaneously.